Digital servo system

Abstract: A digital servo system includes a microcomputer (1) which controls the rotation of a cylinder motor (11) of a video tape recorder. The microcomputer (1) performs functions of a digital filter (6) which has a changeable transfer characteristic as a software. In response to detection that a phase-locked state of the cylinder motor (11) is continuously detected for a predetermined time period, a dynamic range of the digital filter unit (6) is made be gradually larger by changing a coefficient of the digital filter unit (6).

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a digital servo system. More specifically, the present invention relates to a digital servo system which is provided with a digital filter in a servo loop, and a video tape recorder utilizing such a digital servo system. Description of the prior art A digital servo system is used for control a cylinder motor used in a video tape recorder, for example. In the prior art, in order to make a servo gain at a low frequency region in a normal state be high, in general, a digital filter is inserted in a servo loop as a lag filter. If the digital filter is inserted in the servo loop, the servo gain at the low frequency region in the normal state becomes high and thus resistance to disturbance becomes strong; however, there occurs a problem that a phase pull-in time a start of rotation, that is, a step response becomes long. In order to solve such a problem, it can be considered to utilize a technique wherein the digital filter is not inserted in the servo loop at the start of rotation and the same is inserted after that the rotation approaches the normal state; however, in such a case, there is a further problem that discontinuity occurs in a phase error signal and thus a control signal by which the rotation of the rotation body is controlled, before and after the insertion of the filter, and therefore, variation or fluctuation occurs in rotation of the motor. Furthermore, it can be considered to adopt a digital filter having a dynamic range by which the step response and the resistance for the disturbance can be respectively compromised; however, in such a case, there is a further problem that it is impossible to give full display the function of the digital filter as the lag filter. US-A-4878165 discloses a digital servo system according to the precharacterizing part of claim 1. In this system a microprocessor performs a digital filter function based on the output of a speed detector. The output of the speed detector is used to calculate an error signal, which is multiplied by a coefficient which is changed according to the system requirements. On the other hand WO-91/06466 discloses a system for reducing wave and wind induced roll motion of a ship. This system includes an adaptive analog filter. SUMMARY OF THE INVENTION Therefore, a principal object of the present invention is to provide a novel digital servo system. Another object of the present invention is to provide a digital servo system in which a phase pull-in time at a start of rotation is short and resistance to disturbance in a normal state is strong. Another object of the present invention is to provide a video tape recorder in which such a novel digital servo system is utilized. These objects are achieved by a digital servo system according to claim 1 and a video tape recorder according to claim 6 including such a servo system. The dependent claims are related to further advantageous aspects of the present invention. In an aspect of the present invention, the digital servo system further comprises phase-lock detecting means for detecting a phase-lock of the rotation body, and the coefficient changing means changes the coefficient when the phase-lock is continuously detected for a predetermined time period such that the dynamic range of the digital filter is made be gradually larger. In the above describe servo system, a transfer characteristic of the digital filter is changed by changing the coefficient. Accordingly, it becomes possible to insert the digital filter with little influence to the servo system. Especially, if the transfer characteristic is made be flat until the rotation of the rotation body is locked in phase, and thereafter, the dynamic range of the digital filter is made be larger, it is possible to make the step response in the normal state and a time required for locking the servo system be optimum, respectively, and it is possible to minimize unevenness or fluctuation of rotation due to the discontinuity of the control signal. The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a functional block diagram showing one embodiment according to the present invention; Figure 2 is a functional block diagram showing an operation of the embodiment; Figure 3 is a functional block diagram showing a major portion of the embodiment; Figure 4 is a graph showing an operation of the embodiment; Figure 5 is a graph showing an operation of the embodiment, in which only one coefficient can be changed; Figure 6 is a graph showing an operation of the embodiment, in which both coefficient can be changed; Figure 7 is a flowchart showing an operation of the embodiment; Figure 8 is a flowchart showing an operation of the embodiment; Figure 9 is a flowchart showing an operation of the embodiment; Figure 1⊘ is a flowchart showing an operation of the embodiment; Figure 11 is a flowchart showing an operation of the embodiment; and Figure 12 is a graph showing an operation of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, a digital servo system of this embodiment shown includes a microcomputer 1 for performing functions of a speed error signal producing unit 2, a phase error signal producing unit 3, a reference phase signal producing unit 4, an adding unit 5, a digital filter unit 6, a phase-lock detecting unit 7, a coefficient changing unit 8, respectively, in a software manner in accordance with a program. A DAC (Digital to Analog Converter) 9 is constructed as a hardware within the microcomputer 1. A driving signal from the DAC 9 is applied to a driver 1⊘ which drives a cylinder motor 11 of a video tape recorder. In addition, the microcomputer 1 is managed or controlled by a system controller 12 which may be also a microcomputer. An operation of the embodiment will be roughly described with referring to Figure 7 to Figure 12 in the following. Figure 7 shows an operation in a whole. When an FG pulse which is generated according to the rotation of the cylinder motor 11 is inputted to the microcomputer 1, the speed error signal producing unit 2 produces a speed error signal by counting a time interval of the FG pulse (S1). Then, the phase error signal producing unit 3 produces a phase error signal on the basis of a PG pulse which is generated according to the rotation of the cylinder motor 11 and represents a rotation phase of the cylinder motor 11, and a reference phase signal produced by the reference phase signal producing unit 4. That is, the phase error signal is produced by comparing the PG pulse and the reference phase signal with each other by the phase error signal producing unit 3 (S2). In addition, the reference phase signal producing unit 4 produces the reference phase signal by multiplying a vertical synchronization signal (Vsync) which is inputted from an outside when a recording mode is instructed by the system controller 12 of the video tape recorder, and the reference phase signal is inputted by utilizing an internal counter (not shown) when a reproducing or play mode is instructed by the system controller 12. Then, the speed error signal and the phase error signal are added to each other with a predetermined ratio by the adding unit 5 (S3) so as to be inputted to the digital filter unit 6. Such the digital filter unit 6 functions as a lag filter which makes a servo gain at a low frequency region be higher so that the resistance to the disturbance can be made be strong (S4). An operation of the speed error signal producing unit 2 is shown in Figure 8. When the FG pulse is inputted to the microcomputer 1, a value of a reference counter (not shown) which counts a reference clock is read as a value F1. Then, by calculating a difference between the value F1 and a value F2 which has been read in response to a former FG pulse so as to outputs a difference value DSP (S21). Then, the value DSP is compared with STD+STS (S22). If the value DSP is larger than the STD+STS, the value DSP is substituted with a value SMAX which is a maximum value of the speed error signal (S23). In addition, STD means a bias value of the speed error signal, and STS means a changing width of the speed error signal. On the other hands, if the value DSP is smaller that the value STD, the value DSP is substituted with a value SMIN which is a minimum value of the speed error signal (S24, S25). The value F1 is substituted with the value F2, thereby preparing for a next FG pulse input (S27). In addition, if the value DSP is smaller than the value STD+STS but larger than the value STD, the value DSP is substituted with a difference value DSP-STD (S26). Next, operations of the phase error signal producing unit 3 and the phase-lock detecting unit 7 will be described. The PG pulse is generated for each rotation of the cylinder motor 11, representing a rotation phase of the cylinder motor 11. When such a PG pulse is inputted to the microcomputer 1, the phase error signal producing unit 3 read a value of the reference counter as a value P1 and subtracts a reference phase data PREF from the value P1 so as to output a value DPH (S31). In addition, PREF means a value of the reference counter at a timing that the phase reference signal is applied from the phase reference signal producing unit 4. Then, the value DPH is compared with a value PTD+PTS (S32). In addition, PTD means a bias value of the phase error signal, and PTS means a changing width of the phase error signal. If the value DPH is larger than the value PTD+PTS, the value PDH is substituted with a value PMAX that is a maximum value of the phase error signal. If the digital filter unit 6 is to be used in a subsequent stage, "1" is set in a flag LF, and the flag LF is set to be "⊘" when the digital filter unit 6 is not to be used in the subsequent stage (S34), and "⊘" is set in a counter LR which counts a time period during that a phase-locked state continues (S35). If the value DPH is smaller than the value PTD, the value PDH is substituted with a value PMIN that is a minimum value of the phase error signal (S36, S37). Then, "⊘" is set in the flag LF, and "⊘" is set in the time counter LR (S39). If the value DPH is larger that the value PTD but smaller than the value PTD+PTS, the value PDH is substituted with a difference value DPH-PTD (S32, S36, S4⊘), and a counting value of the counter LR is incremented by 1 (S41). Then, it is detected by the phase-lock detecting unit 7 whether or not the value of the counter exceeds "8⊘", for example (S42). If the value of the counter LR becomes to be larger than "8⊘", that is, if it is detected a fact that the phase-lock state is continued for a predetermined time period, "1" is set in the flag LF (S43), and "90" is set in the counter LR (S44). Next, an operation of the adding unit 5 will be described with referring to Figure 1⊘. The speed error signal DSP produced by the speed error signal producing unit 2 and the phase error signal DPH are added to each other with 16:1, and an added result becoming a value DAD (S51). Operations of the digital filter unit 6 and the DAC 9 will be described with referring to Figure 11. If the value of the flag LF is "1", the DAC 9 converts the value DAD which has been processed by the digital filter unit 6 into an analog signal; however, "⊘" is set in the flag LF, the value DAD is converted into an analog signal as it is (S61, S62, S63). In the following, a detailed operation, that is, operations of the digital filter unit 6 and the coefficient changing unit 8 will be described. As shown in Figure 2, the digital filter unit 6 includes a delay unit which constructed by functional components 21 and 22, a coefficient multiplying unit which is constructed by functional components 23, 24 and 25, and an adding unit which is constructed by functional components 26 and 27, in accordance with a difference equation. In Figure 2, the reference symbol X denotes an input signal, the reference symbol Y denotes an output signal, and the reference symbol T denotes a sampling period. In addition, (nT) indicates a value or data at a given timing, and (nT-T) indicates a value or data which is delayed with respect to the data (nT) by the sampling period T. The delay unit delays the signal by the sampling period T. The difference equation can be represented by the following equation (1). Y(nT)= K 3 Y(nT-T) + K 1 X(nT) + K 2 X(nT-T) The equation (1) can be modified as represented by the following equation (2). Y(nT)= Y(nT-T) + a[X(nT)-Y(nT-T)] - b[X(nT)-X(nT-T)] Therefore, the functional block diagram in Figure 2 can be modified as shown in Figure 3. That is, the delay unit includes functional components 31 and 32, the coefficient multiplying unit includes functional components 33 and 34, the adding unit includes a functional component 35, and the subtracting unit includes functional components 36 and 37. In addition, a transfer or frequency characteristic of the digital filter unit 6 can be represented as shown in Figure 4. Then, frequencies f 1 and f 2 , and a gain ratio G can be represented by the following equations (3), (4), and (5). f 1 = a π (2b+a)T f 2 = a π (2-a)T G = 2-a 2b+a = 1 b (b>>a) In addition, "a" and "b" in the equations (3) - (5) are the same as "a" and "b" of the aforementioned equation (2). Furthermore, since the relationship of a>>b is, in general, obtained in a lag filter which is used in the servo system, the equation (5) can be satisfied normally. If the value of the coefficient "a" is fixed and the value of the coefficient "b" is changed in the equation (2), the transfer characteristic of the digital filter unit 6 is changed as shown in Figure 5. If the values of the coefficients "a" and "b", the transfer characteristic of the digital filter unit 6 is changed as shown in Figure 6. Therefore, a type of the digital filter unit of Figure 5 or a type of the digital filter unit of Figure 6 may be selected in accordance with conditions required for respective servo systems. However, in order to made the gain of the servo system be gradually higher after that the phase-locked state continues for the predetermined time period, the type of Figure 5 is preferable. In addition, the above described digital filter unit is constructed by a software, and therefore, repetition of the operation of N times is equal to a series connection of the digital filter units of N, and therefore, whereby the resistance to the disturbance can be made be strong. With referring to Figure 1, if a command for starting the rotation of the cylinder motor 11 is applied to the microcomputer 1 from the system controller 12, the cylinder motor 11 starts rotation. If the transfer characteristic of the digital filter unit 6 is made be flat as shown by F1 in Figure 12 at a timing of the start of rotation of the cylinder motor 11, a time until the phase is locked becomes very short. In addition, as similar to Figure 5 and Figure 6, an ordinate of Figure 12 indicates a gain and an abscissa of Figure 12 indicates a sampling frequency. After the continuity of the phase-locked state is detected by the phase-lock detecting unit 7, if the value of the coefficient "b" is made be gradually smaller by the coefficient changing unit 8, the transfer characteristic of the digital filter unit 6 can be changed to F2, F3, F4, and F5 in Figure 12. More specifically, the gain at a low frequency region is made be gradually larger from F2 to F5, and therefore, the resistance to the disturbance can be made be gradually stronger. In a case where the change of the transfer characteristic of the digital filter unit 6 is made at four stages in the embodiment shown in Figure 12, a degree of the discontinuity of the control signal due to the discontinuity of the transfer characteristic becomes 1/4 in comparison with a case where a digital filter having a transfer characteristic of F5 shown in Figure 13 is suddenly inserted in the servo loop. Therefore, it is possible to insert the digital filter very smoothly. If the number of the stages at which the transfer characteristic of the digital filter 6 is changed is made larger, the continuity of the control signal can be made further large. This is also applied to the type of Figure 6 filter unit. In addition, in a case where it is detected by the phase-lock detecting unit 7 that the continuity of the phase-locked state is broken, that is, a phase-unlock detecting unit detects that the phase becomes an unlocked state, the discontinuity of the phase error signal can be avoided by changing the transfer characteristic of the digital filter unit 6 to F6, F7, F8, F9, and F1⊘ as shown in Figure 12. In contrast, if the digital filter having the transfer characteristic of F6 is suddenly disconnected from the servo loop, the discontinuity of the phase error signal and thus the control signal becomes large; however, according to the embodiment, such a problem can be solved. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the present invention being limited only by the terms of the appended claims.

Claims: A digital servo system for controlling a rotation body, comprising: a servo loop (2, 3, 5, 6, 9, 10, 11) including an error signal producing unit (2, 3) and a digital filter (6) receiving an output of said error signal producing unit (2, 3) and outputting a servo control signal, said digital filter (6) including coefficient multiplying means (33, 34) for multiplying a digital input value with a filter coefficient; characterized in that said digital filter perfors the equation Y(nT) = Y(nT-T) + a [X(nT)-Y(nT-T)]-b[X(nT) -X(nT-T)] wherein Y is the output signal of said digital filter; X the input signal to said digital filter; T the sampling period; n an integer and a, b are the filter coefficients, whereby filter coefficient changing means (8) are provided for changing the coefficients a and b or the coefficient b of the coefficient mulitplying means (33, 34). A digital servo system according to claim 1, wherein the error signal producing means (2, 3) receives speed information and phase information of said rotation body and produces an error signal according to the difference between that speed information and said phase information and a corresponding reference value respectively. A digital servo system according to claim 1, wherein the servo loop further includes a feed means (9) which feeds said servo control signal output from the digital filter (6) to a driving means (10) for the rotation body. A digital servo system according to claim 1, further comprising a phase-lock detection means (7) for detecting a phase-locked state of the rotation body wherein that coefficient changing means changes the coefficient in response to a detection by the phase-lock detecting means. A digital servo system according to claim 1 further comprising a phase-unlock detecting means for detecting if a phase-locked state of the rotation body is broken, wherein said coefficient changing means (8) changes the coefficient in response to the detection of the phase-unlock detecting means. A video tape recorder including a digital servo system according to any of the preceeding claims.

Inventors: TABUCHI JUNICHIRO, TABUCHI, JUNICHIRO
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1994
Main classification: H03H21
IPC codes: H02P29, G11B15